The practice of catching fish or other aquatic organisms usually involves the organisms undergoing some stress. The organisms commonly associate capture with predation and therefore struggle to escape immobilization. This struggle can have a major impact on the post-mortem quality of the tissue of the organism depending upon its duration and at the pre-mortem physical condition of the organism (Lowe, T. E.; Ryder, J. M.; Carragher, J. F.; Wells, R. M. G. 1993: Flesh quality in snapper, Pagrus auratus, affected by capture stress. Journal of Food Science 58: 770-773; and Jerrett, A. R.; Stevens, J.; Holland, A. J. 1995: Tensile properties of rested and exhausted chinook salmon (Oncorhynchus tshaivytscha) white muscle. In press. Journal of Food Science (USA)).
In aquaculture, the cultured organisms are usually individually handled during their life cycle. With excitable fish species such as Chinook Salmon (Oncorhynchus tshawytscha), great care must be taken to ensure that the animals are not bruised, scaled or in any way disfigured or damaged during handling. A natural, undamaged appearance is often a critical factor in determining the final sale price of the fish.
To achieve optimum product quality during harvesting, the organisms must be maintained in a calm state. One approach which has been investigated is the use of anaesthetics during harvesting. Commonly used anaesthetics such as MS-222, 2-phenoxyethanol, benzocaine and more recently, the sedatives etomidate and metomidate (Kreiberg, H. 1992: Metomidate Sedation Minimises Handling Stress in Chinook Salmon. Bulletin of the Aquacultural Association of Canada 92-3: 52-54) have been used to minimise damage during handling but their potential residual toxicity to (or misuse by) humans prevents their use during harvesting.
Non-toxic non-chemical anaesthesia has also been investigated. Commonly used non-toxic alternatives such as cold anaesthesia (Mittal, A. K. and Whitear, M. 1978: A note on cold anaesthesia of poikilotherms. Journal of Fish Biology: 519-520) or carbonic acid anaesthesia (Post, G. 1979: Carbonic Acid Anaesthesia for Aquatic Organisms. The Progressive Fish Culturist 41(3): 142-144) do induce anaesthesia but can also cause considerable trauma in the process. They are accordingly not appropriate for use in harvesting if the quality of the post-mortem flesh is to be maintained as near pre-mortem as is possible.
It is therefore apparent that a need exists for a readily available food grade anaesthetic having low or no toxicity suitable for use inter alia in the harvesting of aquatic organisms. The ideal chemical anaesthesia for harvesting would be cost-effective, have low or non-irritant qualities and be suitable for use with animals intended for human consumption.
The compounds methyl salicylate and ethyl salicylate have been known for many years. Both are liquids which are slightly soluble in water and miscible in alcohol. They have each been employed in a number of similar commercial applications, including in foods, in perfumery and as counterirritants in both human and veterinary medicine. Further, they are both non-toxic at low concentrations and thus are characterisable as "food grade".
It has now surprisingly been found by the applicants that methyl salicylate and ethyl salicylate are capable of being used as aquatic sedatives, anaesthetics and euthanising agents. It is this finding upon which the present invention is based.